The usual video color signal (FBAS-signal) in video recorders is divided into a luminance signal (Y-signal) and a color signal (Chroma-signal). Before it is recorded on a video tape, the luminance signal is limited to a predetermined frequency band, converted to a frequency modulation, and modulated to a predetermined carrier range during this conversion. For recording on a video tape, the color signal is transposed to a frequency range that is below the range of the frequency modulated luminance signal. Each of the video fields to be transmitted is recorded on a helical scan of the video tape. The helical scans are close to each other on the video tape, without any gaps. In a helical scan, the video field lines of adjacent helical scans are arranged in such a way, that the lines of adjacent helical scans are completely parallel to each other. To prevent cross talk between adjacent helical scans during playback, the two magnetic heads, which are provided on a head drum both for recording the video fields in the helical scan, and for playback of the recording, contain a different angle of their head gap with respect to the helical scan, which makes an individual video head only effective for the helical scan assigned to it, and leaves it relatively ineffective for scanning adjacent helical scans. However, this angled position of the head gap in video heads loses its effect in the frequency range in which color is recorded on the video tape.
It is customary to install a comb filter in the color channel on the reproduction side, to compensate for the cross talk that occurs during the scanning of the color signal on the video tape, which, in the simplest case, consists of a delay circuit for delaying one or more video lines, and a signal addition circuit. This comb filter, in conjunction with the repeated sequence of successive lines of the recorded video fields in a different phase relation, erases most of the cross talk signals from the added wanted signals. However, such a comb filter does not prevent an interference streak of a faulty color to appear between the two color surfaces of a line-parallel transition edge of two different colors, caused by the delay of one or more lines in the comb filter. Furthermore, this interference streak expands and is line-parallel displaced with each copy of a recording, so that the color of the playback copy is significantly more faded and blurred than the original recording.
To switch off the color distortion caused by the comb filter in the line-parallel color edges, it is known from DDR Patent 206 521 to detect such a line-parallel color edge with a phase comparator placed parallel to the delay circuit of the comb filter, and, in the event a color transfer has been detected, to separate the second addition input of the addition circuit from the output of the delay circuit. This switches off the distortion effect of the comb filter. The process of the cited publication assumes that the error signal occurring during phase comparison is an immediate measure of the similarity of the color signals. According to the cited publication, this statement only applies when the ratio between the delay time of the delay circuit and the period of the delayed oscillations is a whole number. The effect of the known arrangement is therefore restricted to a very narrow operating range, in which the whole number condition for the phase comparison is at least approximately maintained. This only applies to a small portion of the transmission conditions, so that the picture distortions take place with little change. To reduce the number of picture distortions, the cited patent further proposes to place a subtraction circuit in parallel with the addition circuit of the comb filter, and to essentially supply the larger output signal of the addition or the subtraction circuit to an additional output stage. The known circuit therefore automatically selects a sum or a difference signal as the output signal. With equal phase added, or antiphase subtracted color signals, the thus configured known comb filter transmits the color signal with full comb filter effect through the addition circuit on the output side. This eliminates cross talk disruptions between the adjacent video lines of a video field, even with larger phase deviations, but not, however, the color distortions in horizontal edges of a line-parallel color tone or color intensity transition, caused by the effect of the comb filter. The use of such a comb filter rather increases such line-parallel faulty colors in color transitions, so that the viewer of such a video color picture has to view a considerably more distorted color picture.
For this reason, a various type of comb filter is known from the U.S. Pat. No. 4,931,851. The publication describes a voltage controlled comb filter which comprises a 1 H delay circuit, an inverting circuit, and a mixing circuit. The playback carrier chrominance signal is delayed by the delay circuit for 1 H period and is inverted by the inverting circuit. The playback and delayed playback carrier chrominance signals are inputted to the mixing circuit which mixes these signals in proportion in accordance with the degree of a line correlation error signal. Therefore, there is no switching noise inherent in the conventional comb filter having a switch circuit used in a video type recorder.
The unpublished patent application DE 41 15 213 proposes to connect a detection circuit to the signal outputs of the addition and subtraction circuits of a comb filter, which compares the envelope curves of the addition and the subtraction signals, and creates a control signal from the comparison when the envelope curve of the subtraction signal is larger than the envelope curve of the addition signal, and sets the comb filter circuit, so that the amplitude of the envelope curve of the subtraction signal, which is significant for the comparison in the comparison circuit, during a line-parallel color transition from a white background to a color tone of a predetermined color saturation, is larger than the amplitude of the envelope curve of the addition signal. The proposal assumes that the level of the wanted signals scanned from a video tape is higher than the level of the simultaneously scanned cross talk signals, and that, with the indicated setting of a proposed comb filter circuit, even at a low level of the color signal, the level of the difference signal from the delayed and the undelayed color signal in a color tone or color intensity transition, which can be recognized by the viewer, is nearly always higher than the level of the addition signals simultaneously formed from these color signals. With small color tone and/or color intensity changes in line-parallel transitions, which the viewer can no longer detect, any additional changes in color tone and in color, caused by the comb filter effect, no longer affect the viewer, so that these additional changes in color tone and/or color intensity do not represent any disruptions of the total impression of the reproduced color video picture.